Evo­lu­tion

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Polar Bear Evo­lu­tion Tracked Cli­mate Change

201229Jul21:12

Polar Bear Evo­lu­tion Tracked Cli­mate Change

Infor­ma­tion

pub­lished 29 July 2012 | mod­i­fied 29 July 2012

A whole-​genome analy­sis sug­gests that polar bear num­bers waxed and waned with cli­mate change, and that the ani­mals may have inter­bred with brown bears since becom­ing a dis­tinct species mil­lions of years ago.

Maybe we’re see­ing a hint that in really warm times, polar bears changed their life style and came into con­tact, and indeed inter­bred, with brown bears

Stephan Schus­ter, co-​lead author »

An analy­sis of newly sequenced polar bear genomes is pro­vid­ing impor­tant clues about the species’ evo­lu­tion, sug­gest­ing that cli­mate change and genetic exchange with brown bears helped cre­ate the polar bear as we know it today. The inter­na­tional study, led by the Penn­syl­va­nia State Uni­ver­sity and the Uni­ver­sity at Buf­falo (UB), found evi­dence that the size of the polar bear pop­u­la­tion fluc­tu­ated with key cli­matic events over the past mil­lion years, grow­ing dur­ing peri­ods of cool­ing and shrink­ing in warmer times.

The research also sug­gests that while polar bears evolved into a dis­tinct species as many as 4 – 5 mil­lion years ago, the ani­mals may have inter­bred with brown bears until much more recently. These inti­mate rela­tions may be tied to changes in Earth’s cli­mate, with the retreat of glac­i­ers bring­ing the two species into greater con­tact as their ranges over­lapped, said Char­lotte Lindqvist, the study’s senior author and an assis­tant pro­fes­sor of biol­ogy at UB.

The find­ings have been pub­lished online in the early edi­tion of the Pro­ceed­ings of the National Acad­emy of Sci­ences on July 23. The study is the most exten­sive analy­sis to date of polar bear DNA, sci­en­tists say. The research team, rep­re­sent­ing 13 insti­tu­tions in the U.S., Canada, Europe and Asia, as well as Mexico’s Lab­o­ra­to­rio Nacional de Genom­ica para la Bio­di­ver­si­dad (Lange­bio), sequenced and ana­lyzed the nuclear genomes of 28 dif­fer­ent bears, with many DNA sam­ples pro­vided by the U.S. Geo­log­i­cal Sur­vey and the Nor­we­gian Polar Institute.

“We gen­er­ated a first-​rate set of data, includ­ing deep sequence cov­er­age for the entire genomes of a polar bear, three brown bears and a black bear, plus lower cov­er­age of 23 addi­tional polar bears, includ­ing a 120,000-year-old indi­vid­ual; very few ver­te­brate species have such com­pre­hen­sive genomic resources avail­able,” Schus­ter said. Using this vast amount of data, the sci­en­tists dis­cov­ered that polar bears are actu­ally an older species than pre­vi­ously thought — indeed, far more ancient than sug­gested by a recent study that placed the species’ age at 600,000 years old. That analy­sis looked only at small seg­ments of DNA.

“We showed, based on a con­sid­er­a­tion of the entire DNA sequence, that ear­lier infer­ences were entirely mis­lead­ing,” said study co-​lead author Webb Miller. “Rather than polar bears split­ting from brown bears a few hun­dred thou­sand years ago, we esti­mate that the split occurred 4 – 5 mil­lion years ago.”

She cau­tions, how­ever, that the species’ endurance over sev­eral mil­lion years doesn’t guar­an­tee its future survival.

To model his­tor­i­cal pop­u­la­tions of the polar bear, the sci­en­tists used com­puter sim­u­la­tions to analyse a deeply sequenced polar bear genome. “This is the first time we can see, from their genes, how the pop­u­la­tion his­tory of polar bears tracked Earth’s cli­mate his­tory,” Lindqvist said. “We see an increase in polar bears at the end of the Early Pleis­tocene as the Earth became much colder, and a con­tin­u­ous decline in the size of the pop­u­la­tion dur­ing warmer times.

“We also found, per­haps unsur­pris­ingly, that polar bears occur in much smaller num­bers today than dur­ing pre­his­tory,” Lindqvist con­tin­ued. “They have indeed lost a lot of their past genetic diver­sity, and because of this, they are very likely more sen­si­tive to cli­mate change threats today.”

Dis­crep­an­cies between the esti­mated age of polar bears in the new study and past stud­ies could be explained by inter­breed­ing between polar bears and brown bears since the species split from each other. The new analy­sis uncov­ered more genetic sim­i­lar­i­ties than pre­vi­ously known between polar bears and ABC brown bears, an iso­lated group from south­east­ern Alaska [the ABC Islands, –Moos-​], sug­gest­ing that these ani­mals have exchanged genes since becom­ing sep­a­rate species.

“The ABC brown bears’ mito­chon­dr­ial sequences are much more like polar bears’ than like other brown bears’,” Miller said. “This made us won­der what other parts of their genomes are ‘polar-​bear-​like.’ We mapped such regions, which con­sti­tute 5 to 10 per­cent of their total DNA, onto the genomes of two ABC brown bears. As such, brown/​polar bear hybridi­s­a­tion, which has been observed recently in Arc­tic Canada, has undoubt­edly con­tributed to shap­ing the mod­ern polar bear’s evo­lu­tion­ary story.”

This inter­min­gling between species is just one inter­est­ing find­ing emerg­ing from the enor­mous trove of data that the PNAS study pro­duced. Another ques­tion that the research is begin­ning to address: What makes a polar bear a polar bear? Polar bears have genetic dif­fer­ences from brown bears that let them sur­vive in an Arc­tic cli­mate with very dif­fer­ent diets, and the new study iden­ti­fied genes that may be respon­si­ble for traits such as polar bears’ pig­men­ta­tion and the high fat con­tent of their milk.

The above news item is reprinted from mate­ri­als avail­able at Uni­ver­sity at Buf­falo. Orig­i­nal text may be edited for con­tent and length.